From my experience mullet fishing after dark is pretty crap. You might catch the occasional one, but not as many as you would fishing in the day. I remember many times fishing at the wharf at Timaru, and once it got dark the mullet seemed to disappear, but if you shined a torch at the water there was literally thousands of them swimming past right near the surface, but they didn’t seem to be interested in any bait.
Also what size hooks are you using for the mullet? I use a rig with about 5 or 6 size 10 trout hooks, and just tiny bits of salted bonito or chicken heart, and always seem to have way better luck than other people fishing at the pier. Don’t really know why, I’ve seen other people fishing with really small hooks and small bits of bait, but they never seem to catch much.